The present invention is directed to electrical shift control devices for bicycle transmissions and more particularly, to a motor driven derailleur for use with a bicycle sprocket assembly.
Automatic bicycle transmissions that do not require manual operation by the rider are becoming more desirable. Some automatic transmissions include motor driven derailleurs that operate by electrical switches and/or commands issued by a computer mounted on the bicycle handlebar. For example, a motor driven derailleur may be constructed with a motor mounted to the frame remote from the derailleur, wherein the motor operates the derailleur through a Bowden wire that extends along the frame to the derailleur. This type of motor driven derailleur is very bulky, and the Bowden wire introduces imprecision into the shifting operation that may cause the shifting operation to fail. Another type of motor driven derailleur shown in U.S. Pat. No. 5,480,356 includes a motor that is connected between opposite corners of the parallelogram linkage mechanism of the derailleur, wherein the motor drive shaft in the form of a screw extends and retracts from the motor housing to likewise extend and retract the opposite comers of the linkage mechanism. One difficulty with this type of derailleur is that mud and dirt can build up in the derailleur. The mud and dirt can interfere with the operation of the motor or the derailleur linkage mechanism, and it is very difficult if not impossible to clean.